1. Field of the Invention
The present invention relates to structures of an image data output device and a receiving device by which image data is scaled for scaling up or down an image to be displayed. More specifically, the invention relates to structures of an image data output device and a receiving device combining data for OSD (On Screen Display, the data hereinafter referred to as “OSD data”) with image data to output the resultant composite image.
2. Description of the Background Art
Recent digital broadcasting, for example, employs the “on screen display” technique for displaying, on a screen of a television receiver, an operational menu constituted of characters, graphics and the like superimposed on an image for a broadcast program displayed on the screen. Such OSD data provides an image of an EPG (Electronic Program Guide) for example. Then, an image data output device has recently been developed including an OSD device for displaying any image other than a broadcast program image, that is produced from the OSD data, on a display unit together with the broadcast program image. Image data supplied from this image data output device is required to undergo scaling according to the resolution of the display unit for both of image data for the broadcast program image (hereinafter called “moving image data”) and OSD data.
For a pixel position where the OSD data is provided, the moving image data and OSD data are each weighted and then added together so that the resultant composite data is reproduced.
FIG. 7 shows a structure of a conventional image data output device 5000.
A broadcast signal received by a tuner (not shown) is separated by a demultiplexer (not shown) into moving image data and OSD data to be supplied to image data output device 5000 shown in FIG. 7.
Image data output device 5000 includes a weighting factor multiplier circuit 101 multiplying the moving image data by weighting factor (1−α), a weighting factor multiplier circuit 102 multiplying the OSD data by weighting factor α, a combining circuit 103 adding the moving image data to the OSD data that are multiplied by respective weighting factors by weighting factor multiplier circuits 101 and 102, and a scaling circuit 104 performing scaling on the composite data formed of the moving image data and OSD data produced by combining circuit 103, the scaling being performed according to the resolution of a display unit (not shown) on which the image is reproduced.
It is supposed here that the moving image data and OSD data are to be presented on display units of the same resolution.
In this way, conventional image data output device 5000 as shown in FIG. 7 firstly performs, by weighting factor multiplier circuits 101 and 102 and combining circuit 103, the weighting addition for the moving image data and OSD data. Then, the resultant image data having undergone the weighting addition is scaled according to the resolution of the display unit and then output.
FIG. 8 shows a waveform of the moving image data supplied to weighting factor multiplier circuit 101, and FIG. 9 shows a waveform of the moving image data having been multiplied by the weighting factor by weighting factor multiplier circuit 101 at a pixel position where the OSD data is supplied.
For conventional image data output device 5000 as described above, the moving image data as shown in FIG. 8 is multiplied by the weighting factor by weighting factor multiplier circuit 101 and the resultant moving image data has sharp edges as shown in FIG. 9 on the original moving image data represented by a smooth wave. When the moving image data multiplied by the weighting factor as discussed above undergoes scaling, it is likely that overshoot and undershoot occur at the edges generated on the moving image data. It is thus desirable to perform scaling on the moving image data retaining the smooth wave.
FIG. 10 shows a waveform of the OSD data supplied to weighting factor multiplier circuit 102, and FIG. 11 shows a waveform of the OSD data having been multiplied by the weighting factor by weighting factor multiplier circuit 102.
For the moving image data, the OSD data is provided for displaying characters and the like and has numerous sharp edges as shown in FIG. 10. Therefore, it is desirable to perform scaling on the OSD data having been multiplied by the weighting factor so as to reduce the difference of data amounts of respective edge portions as shown in FIG. 11.
Conventional image data output device 5000 thus suffers from overshoot and undershoot occurring at the edges generated on the moving image data.